denise: early-DDF lo-res picture placement is linear in DDFSTRT

docs/internals/video.md

@@ -36,13 +36,13 @@ visible output one native sample into the fetched stream, so replay
 pre-advances that hidden sample before painting the DIW edge. Extra fetch
 groups from an earlier DDFSTRT are not decoded into the HAM hold colour before
 DIW opens; they are fetched by Agnus, but the first visible HAM history is
-bounded to the display-phase samples. Single-word lo-res fetches that start
-before the standard `$38` DDF slot expose complete 16-pixel groups; the
-standard one-sample phase bias is trimmed when it would push a standard-width
-DIW past the completed early-DDF row at the right edge.
-Late single-word lo-res DDF keeps the standard DIW `$81` one-sample phase;
-the renderer must not subtract an extra sample just to align the clipped
-start to a fetch-unit boundary.
+bounded to the display-phase samples. Single-word lo-res fetch placement is linear in DDFSTRT: each 8-cck fetch
+period before the standard `$38` slot moves the picture exactly 16 lo-res
+pixels left, keeping the standard one-sample phase bias (hardware-verified
+against the vAmigaTS `Agnus/DIW/OLDDIW/diw1` A500 photos, OCS and ECS).
+Early and late single-word lo-res DDF both keep the standard DIW `$81`
+one-sample phase; the renderer must not add or subtract a sample just to
+align the picture to a fetch-unit boundary.
 When DDFSTRT is late enough that DIW opens before DMA has delivered the
 first BPL1DAT word for the row, playfield output remains border-colour until
 that plane-0 fetch reaches Denise instead of sampling stale shifter contents.

src/video/bitplane.rs

@@ -1200,22 +1200,17 @@ impl ControlState {
         // and lost its right edge off the framebuffer.
         let clamped_window_native =
             ((DIW_HSTART_FB0 - diw_h_start as i32).max(0) * 2) / pixel_repeat as i32;
+        // Lo-res FMODE=0 placement is linear in DDFSTRT: moving DDFSTRT one
+        // 8-cck fetch period earlier moves the picture exactly 16 lo-res
+        // pixels left. Real hardware confirms this (vAmigaTS
+        // Agnus/DIW/OLDDIW/diw1 A500 photos: the DDF-$30 stripe grid sits
+        // exactly 16 lo-res pixels left of the standard $38 grid, and the
+        // window-edge staircase steps pair on that grid). An earlier
+        // "-1 sample" phase correction here compensated for the display
+        // window edge sitting one lo-res pixel too far right, which made a
+        // standard DIW overrun the fetched row at the right edge on
+        // early-DDF screens; the picture phase itself was never non-linear.
         let mut origin_shift = display_native_shift - ddf_native_shift + clamped_window_native;
-        let ddf_start = effective_ddf_start_hpos(
-            self.agnus_revision,
-            self.hires() || self.shres(),
-            self.ddfstrt,
-        );
-        if !self.hires() && !self.shres() && self.fetch_quantum() == 1 {
-            let ddf = i32::from(ddf_start);
-            if ddf < standard_ddf && origin_shift > 0 {
-                // Single-word lo-res fetches that start before the standard $38
-                // slot expose whole 16-pixel groups. The standard one-sample
-                // lo-res phase bias must not push a standard-width DIW one sample
-                // past that completed early-DDF row at the right edge.
-                origin_shift -= 1;
-            }
-        }
         // A hi-res/SHRES FMODE=0 screen that starts DDFSTRT earlier than the
         // standard $3C slot pre-fetches whole word(s) before the display window
         // opens. On real hardware those words are shifted into the left border

src/video/bitplane/tests.rs

@@ -1133,9 +1133,38 @@ fn native_x_offset_accounts_for_diw_and_ddf_alignment() {
         lores_early_fetch_standard_window.words_per_row(false, 0),
         21
     );
+    // Linear DDFSTRT placement: $30 fetches one 8-cck period (16 lo-res
+    // pixels) earlier than the standard $38, on top of the standard
+    // one-sample lo-res phase bias. Hardware-verified via the vAmigaTS
+    // Agnus/DIW/OLDDIW/diw1 A500 photos (OCS and ECS).
     assert_eq!(
         lores_early_fetch_standard_window.native_x_offset(false, 2),
-        16
+        17
+    );
+}
+
+#[test]
+fn lores_early_ddf_picture_sits_linearly_left_of_standard_ddf() {
+    // A lo-res FMODE=0 picture fetched at DDFSTRT $30 sits exactly 16 lo-res
+    // pixels (one 8-cck fetch period) left of the same picture fetched at
+    // $38; the DDF->position mapping is linear. Real hardware confirms
+    // (vAmigaTS Agnus/DIW/OLDDIW/diw1 photos: DDF-$30 stripe grid exactly
+    // one fetch period left of the standard grid).
+    let standard = RenderState {
+        bplcon0: 0,
+        diwstrt: ((PAL_VISIBLE_LINE0 as u16) << 8) | STANDARD_DIW_HSTART as u16,
+        ddfstrt: 0x0038,
+        ddfstop: 0x00D0,
+        ..blank_state()
+    };
+    let early = RenderState {
+        ddfstrt: 0x0030,
+        ..standard
+    };
+    assert_eq!(
+        early.native_x_offset(false, 2) - standard.native_x_offset(false, 2),
+        16,
+        "DDFSTRT $30 must sit exactly 16 lo-res px left of $38"
     );
 }
 
@@ -5041,8 +5070,8 @@ fn planned_ham_dma_advances_hold_through_edge_fetch_phase() {
 fn planned_ham_dma_ignores_extra_early_ddf_history_before_diw() {
     let mut row_words = vec![vec![0; 2]; 6];
     row_words[0][0] |= 0x8000; // native x 0: direct palette entry 1
-    row_words[4][0] |= 0x0001; // native x 15: HAM blue := 0
-    row_words[4][1] |= 0x8000; // native x 16: HAM blue := 0
+    row_words[4][1] |= 0x8000; // native x 16: HAM blue := 0 (hidden, pre-DIW)
+    row_words[4][1] |= 0x4000; // native x 17: HAM blue := 0 (first visible)
     let line_plan = DenisePlannedPlayfieldLine::new(0, 64, 66, &row_words, 32);
     let mut control = visible_lowres_control(0x6800);
     control.diwstrt = ((PAL_VISIBLE_LINE0 as u16) << 8) | STANDARD_DIW_HSTART as u16;
@@ -5077,7 +5106,10 @@ fn planned_ham_dma_ignores_extra_early_ddf_history_before_diw() {
         0,
     );
 
-    assert_eq!(control.native_x_offset(control.diw_h_start(), 2), 16);
+    // 17 = one 8-cck fetch period (16 lo-res px) before the standard $38
+    // origin plus the standard one-sample phase bias: DDFSTRT placement is
+    // linear (hardware-verified, vAmigaTS Agnus/DIW/OLDDIW/diw1 photos).
+    assert_eq!(control.native_x_offset(control.diw_h_start(), 2), 17);
     assert_eq!(fb[64], rgb12_to_rgba8(0x0000));
     assert_eq!(fb[65], rgb12_to_rgba8(0x0000));
     assert_eq!(&playfield_mask[64..66], &[0x02, 0x02]);